Using rapid-flow microfluorometry for analysis and sorting of cells, aspects of the following projects have been supported during the previous year: (1) expression of I and Ly antigens on thymocytes; (2) regultory gene control of expression of cell surface antigens coded for by xenotropic murine leukemia virus genomes; (3) characterization of liposome-cell interactions; and (4) early B lymphocyte abnormalities in autoimmune mice.